The invention relates to an anode for a high-temperature fuel cell, in particular for a solid oxide fuel cell, and to the production thereof. The anode is one which is used in a metal substrate-supported high-temperature fuel cell.
Solid oxide fuel cells (SOFC) are high-temperature fuel cells, which are presently operated at operating temperatures of 650 to 1000° C. The gas-tight electrolyte of this cell type comprises a solid ceramic material made of metal oxide, which is able to conduct ions, yet has an insulating effect with respect to electrons. The cathode is generally likewise produced from a ceramic material, which conducts ions and electrons. The anode is produced from a mixture which comprises nickel and yttria-stabilized zirconia, also known as cermet, which likewise conducts ions and electrons.
The development of planar solid oxide fuel cells has resulted in various concepts, which will be briefly described below.
First-generation SOFCs were based on an electrolyte-supported cell concept comprising a relatively thick electrolyte (approximately 150 μm), which was typically composed of yttria-stabilized zirconia (YSZ). Porous electrodes were applied to both sides of this supporting component. The anode generally included a cermet made of metallic and oxidic materials, which frequently were Ni and YSZ. The cathode comprised oxides having a perovskite structure, such as lanthanum strontium manganite (LSM) or lanthanum strontium cobalt ferrite (LSCF).
So as to achieve sufficiently high ionic conductivity of the electrolyte, these fuel cells are operated at temperatures in a narrow interval ranging between 850 and 1000° C. The drawback of these high operating temperatures, however, is the high demands that these entail for the operating process and the materials involved, wherein commonly employed steels cannot be used as interconnectors and heat exchangers because of the high temperatures. The goal has since been to make it possible to operate a high-temperature fuel cell at moderate temperatures, so as to allow the use of less costly materials, without resulting in any loss of performance.
The second-generation SOFCs converted to the so-called anode-supported concept, which allowed operating temperatures of even less than 800° C. to be implemented. Anode-supported fuel cells not only offer more freedom in terms of the stack design, but in addition to a low operating temperature, also create broad latitude between minimum and maximum operating temperatures. An anode-supported fuel cell combines a relatively thick (at a minimum approximately 200 μm, generally 200 to 1500 μm) mechanically load-bearing ceramic anode substrate with a thin, electrochemically active anode functional layer. Both generally comprise a porous nickel/YSZ cermet (YSZ: yttria-stabilized zirconia), to which the now thinner, gas-tight electrolyte is applied. The difference between the substrate and the anode functional layer is frequently not the composition (which is typically nickel and yttria-stabilized zirconia), but usually only the particle size that is used. A gas-tight YSZ electrolyte layer measuring approximately 10 μm is disposed on the anode functional layer. If an LSCF cathode is used in place of LSM, a diffusion barrier made of GCO (gadolinium cerium oxide, or the equivalent thereof gadolinium oxide-doped cerium oxide) is frequently applied between the electrolyte and LSCF cathode, because LSCF and YSZ are not chemically compatible. This diffusion barrier prevents reactions between LSCF and YSZ, notably the formation of electrically insulating intermediate phases.
So as to further improve the operating behavior in terms of thermal cyclability and mechanical stability and to further lower the operating temperatures to 600 to 750° C., the third generation involves electrolytic thin-film systems, which are based on a metallic carrier substrate.
Alternatives also provide for thicker electrolyte layers made of materials having high ionic conductivity (for example gadolinium oxide-doped cerium oxide (GCO), or zirconia fully stabilized with scandium oxide, such as 10Sc1CeSZ). Several metallic alloys, and more particularly certain ferritic steels, exhibit not only thermal expansion that is adjusted well to the cell layers, but also the good long-term behavior required for operating such a fuel cell (for example high corrosion and creep resistance), both when implemented as a dense interconnector and as a porous carrier substrate. At the same time, the classic YSZ/LSM composite cathode was replaced with a double-layer cathode, composed of a cathode layer comprising LSCF and an intermediate layer comprising GCO toward the electrolyte.
Because of the mechanical properties of metallic materials and inexpensive raw materials prices, metal-supported solid oxide fuel cells have great potential in application engineering. For the desired application, a substrate-supported fuel cell should, in the overall, satisfy the following properties and restrictions:
(1) high electronic conductivity of the substrate;
(2) high corrosion stability of the substrate, both in oxidizing and in reducing atmospheres;
(3) a thermal coefficient of expansion of the metallic substrate that is adjusted to the ceramic layers, preferably between 10 and 12·10−6 K−1;
(4) sufficient gas permeability for the fuel gas that is used, which means a porosity of at least 30 to 50% by volume for the substrate; and
(5) reduced surface roughness of the substrate so as to allow level and sealed coatings.
Moreover, the anode should exhibit sufficient catalytic activity in the anode functional layer as well as sufficient mechanical stability and integrity, and notably good adhesion to the substrate surface. The maximum sintering temperature for the applied anode layers should thus be considerably less than 1400° C. in a reducing atmosphere, and more particularly should be around 1200° C.
In particular because of the lower high-temperature resistance of the metallic carriers compared to ceramic carriers, however, different methods must be selected for applying the functional layers of the fuel cell when producing a metal-supported SOFC. The high surface roughness of metallic, porous substrates poses a regular challenge and must be significantly reduced for a functional anode layer and a thin electrolyte layer. In general, the problem of surface roughness can be reduced by way of gradation, in which several powder-technology layers having decreasing particle sizes are employed. Surface roughness has been found to be a particularly critical parameter for methods which can be used to produce dense electrolytes having a small thickness (less than 5 μm) at low temperatures, for example chemical vapor deposition or sol-gel technology.
In the past, thermal spraying and various sintering methods were employed for coating metallic substrates having low temperature resistance with a dense ceramic electrolyte. As a result of the rapid impingement of the molten ceramic particles and sudden cooling of the substrate surface (rapid solidification), thermal spraying generally creates a porous, laminar structure, which exhibits sufficient gas tightness only after several additional layers have been applied. This has the disadvantage of increasing the electrolyte layer thickness from approximately 5 to 10 μm to approximately 40 μm, in comparison with conventional non-metallic, anode-supported fuel cells. The increase in layer thickness of the electrolyte is accompanied by a significant rise in resistance. This resistance is further increased by the pores at the boundaries of the deposited solidification bodies (splats), which until now has prevented power densities to be achieved that are comparable to conventional non-metal-supported fuel cells.
Production by way of a sintering method, which utilizes powders in suspensions or pastes, as with conventional ceramic substrates, and subjects the same to thermal aging for sintering after coating, is limited for metal-supported SOFC primarily by the maximum temperature predetermined by the substrate. The electrolyte materials or powders used for conventional fuel cells comprising ceramic substrates generally require 1350° C. and higher in order to consolidate and form a layer having the required gas tightness. However, in light of the reduced sintering temperatures for metallic substrates, this is no longer feasible. So as to prevent, for example, intermetallic phases in a nickel-containing anode, which impair subsequent operation of the cell, temperatures of no more than 1200° C. are desirable for FeCr alloys used as substrates.
A fuel cell from Ceres Power Ltd. which utilizes a carrier comprising a perforated ferritic steel foil that is approximately 200 to 300 μm thick is known, for example, from [1]. Using conventional methods, such as wet spraying or screen printing, the anode is then deposited as a thick layer made of nickel cermet comprising gadolinium oxide-doped ceria (GCO) in a layer thickness between 10 and 20 μm, while the electrolyte, which likewise comprises GCO, is applied thereon in a layer thickness ranging between 10 and 30 μm by way of an electrophoretic process. Sintering can be carried at temperatures below 1000° C., especially because of the high packing density caused by the electrophoretic process.
The production of a metal-supported SOFC is also disclosed in [2], in which metallic knitted fabrics comprising CroFer22APU and porous plates, produced by way of powder metallurgy, were tested in addition to a nonwoven structure comprising an FeCrAlY alloy, as metallic substrates having a porosity of more than 80% by volume. The Ni/ZrO2 cermet anode, which was approximately 50 μm thick and had a porosity of more than 20% by volume, was plasma sprayed, while DC vacuum plasma spraying, using high-speed nozzles, was employed to produce the dense YSZ electrolyte layer, which was approximately 40 μm thick.
Another method for producing a metal-supported SOFC includes laminating a thin, anode-supported cell onto a thicker metal substrate in the previously sintered state [3]. The drawback of this method is the high complexity of the manufacturing process and adhesion problems of the two components, especially with larger cell geometries. The production of the thin, laminated anode-supported cells alone requires the same technical complexity as conventional anode-supported cells that are already available for use, even without metal substrates. Sintering is carried out at a high temperature (approximately 1400° C.) in an oxidizing atmosphere, which necessitates a different furnace technology than sintering of the metallic component in a reducing atmosphere.
Another option that should be mentioned for applying an electrolyte coating to a metal substrate/anode unit is the PVD (physical vapor deposition, for example sputtering or electron beam evaporation) process, in particular when thin electrolyte layers are desired.